Method and apparatus for transmitting data in a time division duplexed (TDD) communication system

ABSTRACT

Techniques to process data for transmission in a time division duplexed (TDD) communication system. In one aspect, the frequency response of a forward link is estimated at a base station based on reverse link transmissions (e.g., pilots) from a terminal. Prior to a data transmission on the forward link, the base station determines a reverse transfer function based on the pilots transmitted by the terminal, “calibrates” the reverse transfer function with a calibration function to derive an estimate of a forward transfer function, and preconditions modulation symbols based on weights derived from the forward transfer function. In another aspect, the terminal estimates the “quality” of the forward link and provides this information to the base station. The base station then uses the information to properly code and modulate data prior to transmission such that the transmitted data can be received by the terminal at the desired level of performance.

BACKGROUND

1. Field

The present invention relates generally to data communication, and morespecifically to techniques for processing data for transmission in atime division duplexed (TDD) communication system.

2. Background

A multi-channel communication system is often deployed to provideincreased transmission capacity for various types of communication suchas voice, data, and so on. Such a multi-channel system may be amultiple-input multiple-output (MIMO) communication system, anorthogonal frequency division modulation (OFDM) system, a MIMO systemthat utilizes OFDM, or some other type of system. A MIMO system employsmultiple transmit antennas and multiple receive antennas to exploitspatial multiplexing to support a number of spatial subchannels, each ofwhich may be used to transmit data. An OFDM system effectivelypartitions the operating frequency band into a number of frequencysubchannels (or frequency bins or subbands), each of which is associatedwith a respective sub-carrier on which data may be modulated. Amulti-channel communication system thus supports a number of“transmission” channels, each of which may correspond to a spatialsubchannel in a MIMO system, a frequency subchannel in an OFDM system,or a spatial subchannel of a frequency subchannel in a MIMO system thatutilizes OFDM.

The transmission channels of a multi-channel communication systemtypically experience different link conditions (e.g., due to differentfading and multipath effects) and may achieve differentsignal-to-noise-plus-interference ratios (SNRs). Consequently, thetransmission capacities (i.e., the information bit rates) that may besupported by the transmission channels for a particular level ofperformance may be different from channel to channel. Moreover, the linkconditions typically vary over time. As a result, the bit ratessupported by the transmission channels also vary with time.

A time division duplexed (TDD) communication system transmits data onthe forward and reverse links via the same frequency band. The forwardlink refers to transmission from a base station to a terminal and thereverse link refers to transmission from the terminal to the basestation. In the TDD system, the transmission time is partitioned intotime slots, and some of the time slots are allocated for forward linktransmission and remaining time slots are allocated for reverse linktransmission. Because the forward and reverse links share the samefrequency band, the characteristics of the forward link may be estimatedby measuring the characteristics of the reverse link, and vice versa.This reciprocity property of the forward and reverse link propagationmay be used to more easily characterize the communication link.

Given the above, techniques that can be used to (1) exploit thereciprocal property of the forward and reverse links in a TDDcommunication system and (2) process data for transmission on multipletransmission channels with different capacities to achieve highperformance are highly desirable.

SUMMARY

Aspects of the invention provide various techniques to process data fortransmission in a time division duplexed (TDD) communication system. Inone aspect, the frequency response of a forward link is estimated at abase station based on reverse link transmissions (e.g., pilotreferences) from a terminal. Initially, an overall transfer function,H_(f)(ω), of a forward link transmission from the base station to theterminal and an overall transfer function, H_(r)(ω), of a reverse linktransmission on the reciprocal reverse link from the terminal to thebase station are used to derive a calibration function, α(ω), which isdescriptive of the difference between the forward and reverse transferfunctions. Prior to a data transmission on the forward link, the basestation determines the reverse transfer function based on the pilotreferences transmitted by the terminal. The base station then“calibrates” the reverse transfer function with the calibration functionto derive an estimate of the forward transfer function, which is thenused to precondition modulation symbols prior to transmission to theterminal.

In another aspect, the terminal estimates the “quality” of the forwardlink and provides this information to the base station. The forward linkquality may be quantified by a signal-to-noise-plus-interference ratio(SNR), a noise-plus-interference, or some other measurement. The forwardlink quality may be estimated at the terminal based on pilot reference,packet data, or some other signals transmitted on the forward link. Theforward link quality estimate is then represented in a particular formand sent to the base station, which then uses the information toproperly code and modulate data prior to transmission such that thetransmitted data can be received by the terminal at the desired level ofperformance.

The techniques described herein may be applied for data transmission onthe forward and reverse links. The invention further provides methods,systems, and apparatus that implement various aspects, embodiments, andfeatures of the invention, as described in further detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

The features, nature, and advantages of the present invention willbecome more apparent from the detailed description set forth below whentaken in conjunction with the drawings in which like referencecharacters identify correspondingly throughout and wherein:

FIG. 1 is a diagram of a time division duplexed (TDD) communicationsystem capable of implementing various aspects and embodiments of theinvention;

FIG. 2 is a flow diagram of an embodiment of a process to derive acalibration function and to precondition modulation symbols prior totransmission;

FIGS. 3A and 3B show a block diagram of a specific design of atransmitter unit within a base station, which utilizes MIMO and OFDM andis capable of processing data in accordance with an embodiment of theinvention; and

FIG. 4 is a block diagram of a specific design of a receiver unit withina terminal, which is capable of receiving data in accordance with anembodiment of the invention.

DETAILED DESCRIPTION

FIG. 1 is a diagram of a time division duplexed (TDD) communicationsystem 100 capable of implementing various aspects and embodiments ofthe invention. System 100 may be a wireless local area network (LAN) orsome other type of system, and includes a base station 110 incommunication with one or more terminals 150 (only one terminal is shownfor simplicity). System 100 transmits data on the forward and reverselinks in a time division duplexed manner via the same frequency band.For the TDD system, the transmission time is partitioned into timeslots, and some of the time slots are allocated for forward linktransmission and remaining time slots are allocated for reverse linktransmission. For example, the forward and reverse links may beallocated alternating time slots.

The techniques described herein may be applied for data transmission onthe forward and reverse links. However, for clarity, various aspects andembodiments of the invention are specifically described below for theforward link transmission.

System 100 supports one or more transmission channels on each of theforward and reverse links, and some or all of the available transmissionchannels may be used for data transmission at any given moment. Thenumber of transmission channels on the forward link needs not be equalto the number of transmission channels on the reverse link. Multipletransmission channels may be provided via multiple-input multiple-output(MIMO), orthogonal frequency division modulation (OFDM), MIMO incombination with OFDM, or some other constructs. System 100 may alsoimplement code division multiple access (CDMA), time division multipleaccess (TDMA), frequency division multiple access (FDMA), or some othermultiple access techniques. Multiple access techniques can be used tosupport concurrent communication with a number of terminals.

A MIMO system employs multiple (N_(T)) transmit antennas and multiple(N_(R)) receive antennas for data transmission. A MIMO channel formed bythe N_(T) transmit and N_(R) receive antennas may be decomposed intoN_(C) independent channels, with N_(C)≦min {N_(T),N_(R)}. Each of theN_(C) independent channels is also referred to as a spatial subchannelof the MIMO channel and corresponds to a dimension.

An OFDM system effectively partitions the operating frequency band intoa number of (N_(F)) frequency subchannels (i.e., frequency bins orsubbands). At each time slot, a modulation symbol may be transmitted oneach of the N_(F) frequency subchannels. Each time slot corresponds to aparticular time interval that may be dependent on the bandwidth of thefrequency subchannel.

System 100 may be operated to transmit data via one or more transmissionchannels on each of the forward and reverse links. If MIMO is employedbut not OFDM, there is typically only one frequency subchannel and eachspatial subchannel may be referred to as a transmission channel. If OFDMis employed but not MIMO, there is only one spatial subchannel for eachfrequency subchannel and each frequency subchannel may be referred to asa transmission channel. And if both MIMO and OFDM are employed, eachspatial subchannel of each frequency subchannel may be referred to as atransmission channel.

The techniques described herein may be applied to TDD systems employingone or more transmission channels. For clarity, various aspects aredescribed below in which the TDD system employs MIMO and OFDM, althoughneither MIMO nor OFDM is necessary to implement the inventive techniquesdescribed herein.

As shown in FIG. 1, system 100 may be operated to employ a combinationof antenna, frequency, and temporal diversity to increase spectralefficiency, improve performance, and enhance flexibility. In an aspect,base station 110 can be operated to estimate the characteristics of thecommunication link between the base station and terminal and to derivechannel state information (CSI) indicative of the estimated linkcharacteristics. Base station 110 can then be operated to adjust theprocessing (e.g., encoding, modulation, and preconditioning) of dataprior to transmission to terminal 150 based on the CSI derived at thebase station and/or the CSI provided to the base station (e.g., from theterminal).

At base station 110, a data source 112 provides packet data (i.e.,information bits) to a transmit (TX) data processor 114, which (1)encodes the packet data in accordance with a particular encoding scheme,(2) interleaves (i.e., reorders) the encoded data based on a particularinterleaving scheme, (3) channelizes the interleaved data and pilot dataon their respective code channels (if code division multiplexing isused), and (4) maps the channelized packet and pilot data intomodulation symbols for one or more transmission channels used for datatransmission. The encoding increases the reliability of the datatransmission. The interleaving provides time diversity for the codedbits, permits the data to be transmitted based on an averagesignal-to-noise-plus-interference-ratio (SNR) for the transmissionchannels used for the data transmission, combats fading, and furtherremoves correlation between the coded bits used to form each modulationsymbol. The interleaving may further provide frequency diversity if thecoded bits are transmitted over multiple frequency subchannels. Thechannelization allows the packet and pilot data to be separated at theterminal.

In an aspect, the coding, interleaving, and symbol mapping (or anycombination thereof) may be performed based on the CSI available to basestation 110, as indicated in FIG. 1. The encoding, interleaving, andsymbol mapping at base station 110 may be performed based on numerousschemes, some of which are described in further detail below.

A TX channel processor 120 receives and demultiplexes the modulationsymbols from TX data processor 114 and further preconditions themodulation symbols as described below. If MIMO employed, TX channelprocessor 120 provides a stream of preconditioned modulation symbols foreach antenna used for data transmission. If OFDM is employed, TX channelprocessor 120 provides a stream of preconditioned modulation symbolvectors for each antenna used for data transmission, one vector for eachtime slot with each vector including one modulation symbol for eachfrequency subchannel. Each stream is then received and modulated by arespective modulator (MOD) 122 and transmitted via an associated antenna124.

At terminal 150, one or more antennas 152 receive the transmittedsignals and each antenna provides a respective received signal to anassociated demodulator (DEMOD) 154. Each demodulator 154 performsprocessing complementary to that performed at modulator 122. Thereceived modulation symbols from all demodulators 154 are then providedto a receive (RX) channel processor 156, which performs receiverprocessing complementary to the processing performed by TX channelprocessor 120. RX channel processor 156 provides recovered modulationsymbols to a RX data processor 158, which processes the symbols torecover the transmitted data streams. RX data processor 158 performsprocessing complementary to that performed by TX data processor 114. Thedecoded data is then provided to a data sink 160. The processing by basestation 110 and terminal 150 is described in further detail below.

The one or more transmission channels available to system 100 typicallyexperience different link conditions (e.g., due to different fading andmultipath effects) and may achieve different SNRs. Consequently, thecapacity of the transmission channels may differ from channel tochannel. This capacity may be quantified by the information bit rate(i.e., the number of information bits per modulation symbol) that may betransmitted on each transmission channel for a particular level ofperformance (e.g., a particular bit error rate (BER) or packet errorrate (PER)). Since the link conditions typically vary with time, thesupported information bit rates for the transmission channels also varywith time.

To more fully utilize the capacity of the transmission channels, channelstate information (CSI) descriptive of the forward link may bedetermined and used to process (e.g., encode, modulate, andprecondition) data such that the transmitted information bit rates matchthe transmission capacities of the transmission channels. The CSI mayinclude various types of information and may be derived and/or providedin various forms, some of which are described below.

One type of CSI relates to the “quality” of the forward link. Thisquality may be quantified by the SNRs of individual transmissionchannels or an average SNR of each group of transmission channels, therates or average rate supported by the channels, and coding andmodulation schemes supported by the channels, and so on, as describedbelow. Information descriptive of the forward link quality may be usedto properly code and modulate data prior to transmission such that thetransmitted data may be recovered by the terminal with the desired levelof performance (1% PER). Estimation and use of the forward link qualityis described in further detail below.

Another type of CSI relates to the “response” of the forward link. Thisresponse may be quantified by the amplitude and phase across the entireoperating frequency band for the propagation path between eachtransmit-receive antenna pair used for data transmission. Informationdescriptive of the forward link response may be used to preconditionmodulation symbols prior to transmission to orthogonalize the spatialsubchannels, which may improve performance. Characterization of theforward link response and preconditioning of the modulation symbols arealso described in further detail below.

Various types of transmission may be used to characterize the forwardlink response and to estimate the forward link quality. For example,pilot data (i.e., data of a known pattern such as a sequence of allzeros), packet data, signaling, and possibly other types of transmissionmay be used. For clarity, various aspects and embodiments of theinvention, including the characterization of the forward link responseand the estimation of the quality of the forward link, are describedbelow based on the use of pilot reference.

Characterization of Forward Link Response

For a TDD communication system, a single frequency band is used for boththe forward and reverse links, and the propagation paths for the forwardand reverse links are reciprocal. In this case, the characteristics ofthe forward link may be estimated based on measurements of the reverselink, and vice versa, if the time-variant changes in the communicationlink are slow relative to the difference between the time the link isestimated and the time the estimates are used. For example, if theforward and reverse links are assigned alternating time slots, then thetime slots should be short enough so that the communication link doesnot change appreciably between the time slot in which the link ischaracterized and the time slot in which the link characterization isapplied for a data transmission.

In accordance with an aspect of the invention, the response of theforward link is estimated at the base station based on reverse linktransmissions from the terminal. An overall transfer function, H_(f)(ω),of a forward link transmission from the base station to the terminal andan overall transfer function, H_(r)(ω), of a reverse link transmissionon the reciprocal reverse link from the terminal to the base station maybe expressed as:H _(f)(ω)=T _(f)(ω)C(ω)R _(f)(ω),andH _(r)(ω)=T _(r)(ω)C(ω)R _(r)(ω),  Eq (1)where

-   -   T_(f)(ω) is a transfer function for the aggregate processing at        the base station for the forward link transmission (e.g., the        transfer function for TX channel processor 120 and modulator 122        within base station 110 in FIG. 1);    -   R_(f)(ω) is a transfer function for the aggregate processing at        the terminal for the forward link transmission (e.g., the        transfer function for demodulator 154 and RX channel processor        156 within terminal 150 in FIG. 1);    -   C(ω) is the channel frequency response (e.g., for a particular        propagation path or transmit-receive antenna pair);    -   T_(r)(ω) is a transfer function for the aggregate processing at        the terminal for the reverse link transmission (e.g., the        transfer function for TX channel processor 162 and modulator 154        within terminal 150 in FIG. 1); and    -   R_(r)(ω) is a transfer function for the aggregate processing at        the base station for the reverse link transmission (e.g., the        transfer function for demodulator 122 and RX channel processor        132 within base station 110 in FIG. 1).

H_(r)(ω) may be determined at the base station based on pilot referencetransmitted from the terminal on the reverse link. Similarly, H_(f)(ω)may be determined at the terminal based on pilot reference transmittedfrom the base station on the forward link, and may be subsequentlyprovided to the base station.

As noted above, the forward and reverse links for a TDD system aregenerally reciprocal. Thus, if the signal processing (e.g., filtering)at the base station and terminal for the forward link transmission isidentical to the signal processing at the terminal and base station forthe reverse link transmission, then both the base station and terminalcan measure identical transfer functions (i.e., H_(f)(ω)=H_(r)(ω)),except for errors induced by estimation and imperfect calibration.However, in a practical implementation, the forward transfer function,H_(f)(ω), may not be identical to the reverse transfer function,H_(r)(ω). This may be due to, for example, different signal processingelements used at the base station and terminal for the forward andreverse link transmissions. For example, the frequency response of thetransmit and receive filters for the forward link transmission may bedifferent from the frequency response of the transmit and receivefilters for the reverse link transmission.

In accordance with an aspect of the invention, the difference betweenthe forward and reverse transfer functions for the forward and reverselink transmissions is determined and used to more accurately estimatethe forward transfer function at the base station, which may provideimproved performance. Initially, the base station determines the reversetransfer function, H_(r)(ω), based on the pilot reference transmitted bythe terminal. The terminal also determines the forward transferfunction, H_(f)(ω), based on the pilot reference transmitted by the basestation. The terminal then sends information indicative of the forwardtransfer function, H_(f)(ω), back to the base station, which then usesthe information to perform a calibration. Since the signal processing atthe base station and terminal typically does not change appreciablyduring a communication session, H_(f)(ω), may be reported at the startof the session and may be updated thereafter, as necessary.

A calibration function, α(ω)), may then be derived based on the forwardand reverse transfer functions, as follows: $\begin{matrix}\begin{matrix}{{a(\omega)} = \frac{H_{f}(\omega)}{H_{r}(\omega)}} \\{= \frac{{T_{f}(\omega)}{C(\omega)}\quad{R_{f}(\omega)}}{{T_{r}(\omega)}\quad{C(\omega)}\quad{R_{r}(\omega)}}} \\{= {\frac{{T_{f}(\omega)}\quad{R_{f}(\omega)}}{{T_{r}(\omega)}\quad{R_{r}(\omega)}}.}}\end{matrix} & {{Eq}\quad(2)}\end{matrix}$In equation (2), it is assumed that there are no zeros in the reversetransfer function, H_(r)(ω), so that division by zero is notencountered. The calibration function, α(ω)), typically includes a setof complex values for a set of frequencies.

If MIMO is employed, then the calibration function may be derived basedon the pilot transmitted on a particular propagation path (i.e., aparticular transmit-receive antenna pair). If OFDM is employed, then thecalibration function may be derived based on the pilot transmitted onall or a subset of the N_(F) frequency subchannels. In this case, thecalibration function, α(k), may be expressed as a function of k, theindex of the frequency subchannels and would typically include N_(F)complex values for the N_(F) frequency subchannels.

The signal processing at both the base station and terminal may be afunction of frequency, and a calibration function is typically derivedfor each propagation path (i.e., each transmit-receive antenna pair).The calibration function for propagation path (i, j, k) (i.e., the pathfor the k-th frequency subchannel from the i-th terminal antenna to thej-th base station antenna) may be expressed as:${a\left( {i,j,k} \right)} = {\frac{{T_{f}\left( {j,k} \right)} \cdot {R_{f}\left( {i,k} \right)}}{{T_{r}\left( {i,k} \right)} \cdot {R_{r}\left( {j,k} \right)}}.}$For example, if there are two transmit antennas and two receiveantennas, then four calibration functions may be derived, one for eachof the four transmit-receive antenna pairs.

Prior to a data transmission from the base station to the terminal, theforward link response may be characterized at the base station based onthe pilot reference transmitted on the reverse link. In an embodiment,the forward link response characterization entails estimating theresponse of each transmit-receive antenna pair of each frequencysubchannel (i.e., each propagation path). A reverse transfer functionh_(r)(i, j, k) for propagation path (i, j, k) may be expressed as:h _(r)(i, j, k)=T _(r)(i, k)·c(i, j, k)·R _(r)(j, k).This reverse transfer function h_(r)(i, j, k) may be obtained byprocessing and measuring the pilot reference transmitted on propagationpath (i, j, k).

The base station can then estimate the forward transfer function,h_(f)(i, j, k), for propagation path (i, j, k) by multiplying thereverse transfer function, h_(r)(i, j, k), with the calibrationcoefficient, α(i, j, k), as follows: $\begin{matrix}\begin{matrix}{{{h_{f}\left( {i,j,k} \right)} = {{h_{r}\left( {i,j,k} \right)} \cdot {a\left( {i,j,k} \right)}}},} \\{{= {{T_{r}\left( {i,k} \right)} \cdot {c\left( {i,j,k} \right)} \cdot {R_{r}\left( {j,k} \right)} \cdot \frac{{T_{f}\left( {j,k} \right)} \cdot {R_{f}\left( {i,k} \right)}}{{T_{r}\left( {i,k} \right)} \cdot {R_{r}\left( {j,k} \right)}}}},} \\{= {{T_{f}\left( {j,k} \right)} \cdot {c\left( {i,j,k} \right)} \cdot {{R_{f}\left( {i,k} \right)}.}}}\end{matrix} & {{Eq}\quad(3)}\end{matrix}$

The forward transfer functions, h_(f)(i, j, k), for all propagationpaths may then be used to form one or more channel response matricesH_(f)(k). Typically, one channel response metric H_(f)(k) is formed foreach frequency subchannel. Each channel response matrix H_(f)(k)comprises an N_(T)×N_(R) matrix of forward transfer functions h_(f)(i,j, k), where 1≦i≦N_(R) and 1≦j≦N_(T), for the N_(T)-N_(R)transmit-receive antenna pairs for the k-th frequency subchannel. Eachchannel response matrix H_(f)(k) may then be used to derive weights,which are then used to precondition the modulation symbols for the k-thfrequency subchannel, as described below.

For each frequency subchannel, an eigenvector decomposition of aHermitian matrix formed by the product of the channel response matrixH_(f) with its conjugate-transpose H_(f) ^(H) can be expressed as:H _(f) ^(H) H _(f) =EΛE ^(H),  Eq (4)where E is an eigenvector matrix and Λ is a diagonal matrix ofeigenvalues, both of dimension N_(T)×N_(T), and the symbol “H” denotesthe conjugate-transpose. The base station preconditions a vector ofN_(C) modulation symbols, b, for each time slot using the eigenvectormatrix E, as follows:x=Eb.  Eq (5)The preconditioning in equation (5) can be more fully expressed as:$\begin{matrix}{{\begin{bmatrix}x_{1} \\x_{2} \\\vdots \\x_{N_{T}}\end{bmatrix} = {\begin{bmatrix}{e_{11},} & {e_{12},} & \cdots & e_{1N_{C}} \\{e_{21},} & {e_{22},} & \quad & e_{2N_{C}} \\\vdots & \quad & ⋰ & \vdots \\{e_{N_{T}1},} & {e_{N_{T}1},} & \cdots & e_{N_{T}N_{C}}\end{bmatrix} \cdot \begin{bmatrix}b_{1} \\b_{2} \\\vdots \\b_{N_{C}}\end{bmatrix}}},} & {{Eq}\quad(6)}\end{matrix}$where

-   -   [b₁, b₂, . . . b_(N) _(C) ] are the modulation symbols for        spatial subchannels 1, 2, . . . N_(C), respectively, where        N_(C)≦min {N_(T), N_(R)};    -   e_(ij) are elements of the eigenvector matrix E, which is        descriptive of the transmission characteristics from the base        station antennas to the terminal antennas; and    -   [x₁, x₂, . . . x_(N) _(T) ] are the preconditioned modulation        symbols, which can be expressed as:        x ₁ =b ₁ ·e ₁₁ +b ₂ ·e ₁₂ + . . . +b _(N) _(C) ·e _(1N) _(C) ,        x ₂ =b ₁ ·e ₂₁ +b ₂ ·e ₂₂ + . . . +b _(N) _(C) ·e _(2N) _(C) ,        and        x _(N) _(T) =b ₁ ·e _(N) _(T) ₁ +b ₂ ·e _(N) _(T) ₂ + . . . +b        _(N) _(C) ·e _(N) _(T) _(N) _(C) .        Since H^(H)H is Hermitian, the eigenvector matrix E is unitary.        Thus, if the elements of the vector b have equal power, the        elements of the vector x (i.e., the preconditioned modulation        symbols) also have equal power.

As shown in equations (5) and (6), for each frequency subchannel, theN_(C) modulation symbols at each time slot are preconditioned togenerate N_(T) preconditioned modulation symbols. The preconditioningorthogonalizes the modulation symbol streams transmitted on the N_(C)spatial subchannels, which may improve performance.

FIG. 2 is a flow diagram of a process to derive the calibration functionand to precondition modulation symbols prior to transmission on theforward link, in accordance with an embodiment of the invention.Initially (e.g., at the start of a communication session between a basestation and a terminal), the terminal measures the forward transferfunction, H_(f)(ω), and sends it back to the base station, at step 212.The base station also measures the reverse transfer function, H_(r)(ω),at step 214. The forward and reverse transfer functions, H_(f)(ω) andH_(r)(ω), are then used by the base station to derive the calibrationfunction, α(ω), as shown in equation (2), at step 216. Steps 212 through216 may be performed prior to the first data transmission from the basestation to the terminal, and may also be performed thereafter as neededduring the communication session.

A determination is then made whether or not there is a data transmissionfor the terminal, at step 218. If there is no data to transmit, then theprocess returns to step 218 and waits (e.g., until the next time slotfor the forward link). Otherwise, if there is data to transmit, the basestation measures the reverse transfer function, H_(r)(ω)), at step 220.The base station then derives the forward transfer function, H_(f)(ω),based on the reverse transfer function, H_(r)(ω), and the calibrationfunction, α(ω), as shown in equation (3), at step 222. The forwardtransfer function, H_(f)(ω), is then used to process data prior totransmission to the terminal, at step 224. More particularly, theforward transfer function, H_(f)(ω), is used to derive the eigenvectormatrix E, as shown in equation (4), which is then used to preconditionthe modulation symbols, as shown in equations (5) and (6). The processthen returns to step 218 and waits for the next data transmission (orthe next time slot).

Receiver Processing

The received signals at the terminal may be expressed as:$\begin{matrix}\begin{matrix}{{\underset{\_}{r} = {{H_{f}\underset{\_}{x}} + \underset{\_}{n}}},} \\{{= {{H_{f}E\quad\underset{\_}{b}} + \underset{\_}{n}}},}\end{matrix} & {{Eq}\quad(7)}\end{matrix}$where n is the channel noise.

To recover the transmitted modulation symbols, the terminal initiallyperforms a channel-matched-filter operation, followed by amultiplication by the right eigenvector matrix. The result of thechannel-matched-filter and multiplication operations is a vector y,which can be expressed as: $\begin{matrix}\begin{matrix}{{\underset{\_}{y} = {E^{H}H_{f}^{H}\underset{\_}{r}}},} \\{{= {{E^{H}H_{f}^{H}H_{f}E\quad\underset{\_}{b}} + {E^{H}H_{f}^{H}\underset{\_}{n}}}},} \\{{= {{\Lambda\underset{\_}{b}} + \underset{\_}{\hat{n}}}},}\end{matrix} & {{Eq}\quad(8)}\end{matrix}$where {circumflex over (n)} is the noise term after the receiverprocessing at the terminal. The channel response matrix, H_(f), for theforward link used to perform the channel-matched-filter operation may bedetermined at the terminal based on the pilot reference transmitted fromthe base station. As shown in equation (8), the resultant vector y is ascaled version of the vector b, which includes the modulation symbolsprior to the preconditioning with the eigenvector matrix E at the basestation. The scaling between the vectors b and y is based on theeigenvalues in the matrix Λ.

The noise term {circumflex over (n)} after the receiver processing has acovariance that can be expressed as: $\begin{matrix}\begin{matrix}{{{E\left\{ {\underset{\_}{\hat{n}}\quad{\underset{\_}{\hat{n}}}^{H}} \right\}} = {E\left( {E^{H}H_{f}^{H}\underset{\_}{n}\quad{\underset{\_}{n}}^{H}H\quad E} \right)}},} \\{{= {E^{H}H_{f}^{H}H_{f}E}},} \\{{= \Lambda},}\end{matrix} & {{Eq}\quad(9)}\end{matrix}$where E{} is the expectation operation. From equation (9), the noisecomponents for the spatial subchannels, after the receiver processing atthe terminal, are independent with variances given by the eigenvalues.If the elements of b have equal power and if E{{circumflex over(n)}{circumflex over (n)}^(H)}=I, then the SNR of the i-th component ofthe vector y (i.e., the i-th spatial subchannel) is λ_(i), which is thei-th diagonal element of the matrix Λ of eigenvalues.

Pilot Transmission Schemes

Various schemes may be used to transmit a pilot reference from atransmitter unit (e.g., a terminal) to a receiver unit (e.g., a basestation) to allow the receiver unit to characterize the communicationlink. The pilot reference is typically transmitted from all or a subsetof the antennas at the transmitter unit and on a sufficient number offrequency subchannels such that an accurate characterization of thecommunication link can be made. The transmitted pilot reference isreceived by all antennas at the receiver unit and processed tocharacterize the link response.

The particular pilot transmission scheme to be used may be selectedbased on various system constraints and considerations. The transmissionscheme used for the reverse link may not be the same as that used forthe forward link. As an example, the forward link may support MIMOwhereas the reverse link may only support single-input multiple-output(SIMO), in which case the pilot transmission scheme should account forthis difference. Some schemes for transmitting the pilot reference onthe reverse link from the terminal to the base station are describedbelow.

In one pilot transmission scheme, the terminal transmits the pilotreference continuously (i.e., in all time slots allocated for thereverse link) from all antennas and on a sufficient number ofsubchannels. The pilot reference may be transmitted along with otherpacket data. For this scheme, the pilot reference is readily availableto the base station for channel characterization at all times.

If the pilot reference is transmitted from multiple antennas on aparticular frequency subchannel (there may be only one frequencysubchannel if OFDM is not employed) in a given time slot, then the pilottransmitted on different antennas should be orthogonal to each other.This would then allow the base station to recover the individual pilotreference transmitted on each antenna. In a single-carrier system,orthogonality may be achieved by transmitting the pilot on a differentcode channel from each antenna (e.g., covering the pilot data for eachantenna with a different Walsh code).

If OFDM is employed, the frequency response of the communication linkmay be determined for a subset of the frequency subchannels instead ofall frequency subchannels. For example, if N_(F) frequency subchannelsand N_(R) antennas are available at the terminal, then the pilotreference may be transmitted on N_(F)/N_(R) frequency subchannels ofeach of the N_(R) antennas. The N_(F)/N_(R) frequency subchannels usedfor pilot transmission on each antenna are selected such that anaccurate estimate of the frequency response may be made. For example,the pilot reference may be transmitted on frequency subchannels that arespaced over the entire frequency band and are N_(R) channels apart. TheN_(F)/N_(R) frequency subchannels for each antenna may be staggered(i.e., offset) relative to those of other antennas. In general, thenumber of frequency subchannels selected for pilot transmission for eachantenna should be sufficient to allow for an accurate sampling of thespectrum of the communication link. The frequency domain sampling allowsthe forward link response to be estimated using fewer pilottransmissions.

In another pilot transmission scheme, the terminal transmits a pilotreference in a time division multiplexed (TDM) manner from all antennasand on all or a subset of the frequency subchannels. For example, thepilot reference may be transmitted on all or a subset of the frequencysubchannels from one antenna at each available time slot in a sequentialmanner (e.g., in a round-robin manner). Thus, the pilot reference may betransmitted from antenna 1 at a particular time slot, from antenna 2 atthe next available time slot, and so on. Numerous other TDM schemes mayalso be used to transmit the pilot reference from the terminal.

In yet another pilot transmission scheme, multiple terminals are able toconcurrently transmit pilot references at the same time via codedivision multiplexing (CDM). For example, terminal 1 may transmit thepilot reference on a first set of code channels (from all or a subset ofthe antennas), terminal 2 may transmit the pilot reference on a secondset of code channels, and so on. The code channels are designed to beorthogonal to each other so that the pilot references from differentterminals can be individually recovered at the base station, which thenallows the base station to characterize the forward link response foreach of the terminals.

In yet another pilot transmission scheme, multiple terminals are able toconcurrently transmit pilot references at the same time via timedivision multiplexing (TDM). In an implementation, one terminal may bedesignated to transmit pilot reference on the reverse link at any giventime slot. In a first embodiment, the pilot transmissions on the reverselink are scheduled by the base station using a scheme such as polling.Whenever the base station needs to transmit data to a particularterminal, that terminal is notified prior to the data transmission. Theterminal then transmits pilot reference on all or a subset of thefrequency subchannels and from all or a subset of the antennas to allowthe base station to characterize the forward link for this terminal. Ina second embodiment, the terminals are assigned fixed time slots forpilot transmission. For example, terminal 1 may be assigned to transmitthe pilot reference in every n-th time slot starting with a particulartime slot (from all or a subset of the antennas and on all or a subsetof the frequency subchannels), terminal 2 may be assigned to transmitthe pilot reference on every n-th time slot starting with the nextavailable time slot, and so on. For TDM pilot transmission schemes, thetime slots are assigned to the terminals such that the forward link canbe characterized as close to the time of actual data transmission on theforward link. In this way, accurate forward link characterization may beobtained.

In yet another pilot transmission scheme, multiple terminals are able toconcurrently transmit pilot references at the same time via frequencydivision multiplexing (FDM). For example, at time slot n, terminal 1 maytransmit the pilot reference on frequency subchannel 1 (from all or asubset of the antennas), terminal 2 may transmit the pilot reference onfrequency subchannel 2, and so on.

As described above, the forward link frequency response for eachterminal may be sampled using a subset of the frequency subchannels. Forexample, N_(X) terminals may be designated to transmit pilot referenceson N_(F) frequency subchannels, and each terminal may be allocatedN_(F)/N_(X) frequency subchannels for pilot transmission. TheN_(F)/N_(X) frequency subchannels for each terminal are selected suchthat an accurate estimate of the frequency response may be made. Thepilot reference may be transmitted on frequency subchannels that arespaced over the entire frequency band and are N_(X) channels apart.

Using the techniques described above, the base station is able tocharacterize the response of the forward link based on the pilotreference transmitted by the terminal. This characterization typicallyincludes estimates of the frequency response for the communication link(e.g., the MIMO channel). This information may be used to orthogonalizethe spatial subchannels, as described above.

In some other embodiments, the forward link is characterized at theterminal based on the pilot transmitted from the base station. Theforward link characterization is then sent back to the base station,which then uses the information to process data (e.g., precondition themodulation symbols) for the forward link transmissions.

Forward Link quality Estimate/Coding and Modulation Scheme

In accordance with another aspect of the invention, the terminalestimates the “quality” of the forward link and provides thisinformation to the base station. The forward link quality may bequantified by the SNR, the noise-plus-interference, or some othermeasurement. The SNR may be estimated for each transmission channel,each transmit-receive antenna pair, or for groups of these transmissionchannels or antenna pairs. The forward link quality may be estimatedbased on the pilot reference, data, or some other signals transmitted onthe forward link. The forward link quality estimate is then representedin a particular form and sent to the base station, which then uses theinformation to properly code and modulate data prior to transmissionsuch that the transmitted data can be received by the terminal at thedesired level of performance (e.g., 1% PER).

In an embodiment, one coding and modulation scheme is used for alltransmission channels to be used for the forward link transmission. Inthis case, the terminal can estimate the average SNR across alltransmission channels and report this average SNR to the base station.For example, the terminal can process the pilot reference received oneach antenna and estimate the SNR for each propagation path, and mayfurther derive the average SNR from the individual SNRs. Reporting theaverage SNR instead of the individual SNRs can significantly reduce theamount of information to be sent back to the base station.

In another embodiment, a number of data streams may be transmitted on anumber of groups of transmission channels. Each data stream may beindependently processed (i.e., coded and modulated) with a particularcoding and modulation scheme, or multiple data streams may share acommon coding and modulation scheme. Each group of channels may includeany number and type of transmission channels. For example, one group maybe defined for each antenna at the base station, with each groupincluding all frequency subchannels for the associated antenna. Asanother example, one group may be defined for each frequency subchannel,with each group including all spatial subchannels for the associatedfrequency subchannel. As yet another example, one group may be definedfor each different subset of frequency subchannels, with each groupincluding all spatial subchannels for the associated frequencysubchannels. Alternatively, each group may include one transmissionchannel.

The forward link quality may be reported to the base station via variousforms. In an embodiment, the SNRs for the individual transmissionchannels or an average SNR for each group of channels may be determinedand reported. In another embodiment, the terminal can map the estimatedSNR to a particular rate (e.g., a data rate indicator (DRI)) and sendthe rate information back to the base station. The rate indicator may beindicative of the maximum data rate that may be transmitted on thecorresponding group of transmission channels for the required level ofperformance. In yet another embodiment, the terminal may report anindication of a particular coding and modulation scheme to be used bythe base station for each data stream to be independently processed. Inyet another embodiment, the noise or interference power for all or asingle frequency subchannel, the average channel noise variance, or someother measurement, may be reported. Various other forms may also be usedto report the forward link quality and are within the scope of theinvention. In general, the terminal may report any type of informationthat can be used at the base station to properly code and modulate dataprior to transmission on the forward link.

The terminal can thus estimate the forward link quality for each groupof transmission channels and report information indicative of theestimated quality (e.g., the average SNR, a rate indicator, or anindication of a coding and modulation scheme) to the base station. Byreporting information for each group, the base station is able toindividually and independently code and modulate each data stream basedon the received information for the associated group of transmissionchannels, which may provide improved performance. For example, the basestation may be able to transmit different data streams on differentantennas, with each data stream having a particular bit rate supportedby the transmission channels used to transmit the data stream.

In certain embodiments, the forward link quality can be estimated at thebase station based on the pilot reference transmitted by the terminal.Because the forward and reverse links are reciprocal, the quality of theforward link can be estimated based on the quality of the reverse link,which can be estimated by the base station. Initially, the base stationcan estimate the SNR (e.g., average SNR) for the reverse link based onthe pilot reference transmitted by the terminal (i.e., the same pilotreference used to derive the reverse channel response matrix H_(r) atthe base station). The base station is able to estimate the channelnoise variance.

As shown in equation (8), the vector z recovered at the terminal isequal to the vector b scaled by the eigenvalues in the matrix Λ. Thematrix Λ can also be derived at the base station based on the forwardchannel response matrix H_(f). The base station can thus estimate thesignal power at the terminal (based on the matrix Λ and the vector b).The SNR at the terminal is then estimated based on the signal power andthe channel noise variance computed at the base station. The estimatedSNR can then be used to properly code and modulate the data.

For the embodiments wherein the forward link quality is estimated at thebase station, the coding and modulation scheme(s) selected for use bythe base station may be sent to the terminal so that it can use thecorresponding demodulation and decoding scheme(s) to recover thetransmitted data. Alternatively, the terminal may perform “blind”decoding whereby it processes the received signals based on a number ofhypotheses corresponding to a number of possible coding and modulationschemes.

Various techniques may be used to estimate the link quality. Some ofthese techniques are described in the following patents, which are allassigned to the assignee of the present application and incorporatedherein by reference:

-   -   U.S. Pat. No. 5,799,005, entitled “SYSTEM AND METHOD FOR        DETERMINING RECEIVED PILOT POWER AND PATH LOSS IN A CDMA        COMMUNICATION SYSTEM,” issued Aug. 25, 1998,    -   U.S. Pat. No. 5,903,554, entitled “METHOD AND APPARATUS FOR        MEASURING LINK QUALITY IN A SPREAD SPECTRUM COMMUNICATION        SYSTEM,” issued May 11, 1999,    -   U.S. Pat. Nos. 5,056,109, and 5,265,119, both entitled “METHOD        AND APPARATUS FOR CONTROLLING TRANSMISSION POWER IN A CDMA        CELLULAR MOBILE TELEPHONE SYSTEM,” respectively issued Oct. 8,        1991 and Nov. 23, 1993, and    -   U.S. Pat. No. 6,097,972, entitled “METHOD AND APPARATUS FOR        PROCESSING POWER CONTROL SIGNALS IN CDMA MOBILE TELEPHONE        SYSTEM,” issued Aug. 1, 2000.

Methods for estimating a single transmission channel based on a pilotreference or a data transmission may also be found in a number of papersavailable in the art. One such channel estimation method is described byF. Ling in a paper entitled “Optimal Reception, Performance Bound, andCutoff-Rate Analysis of References-Assisted Coherent CDMA Communicationswith Applications,” IEEE Transaction On Communication, October 1999, andincorporated herein by reference.

Transmitter Unit At the Base Station

FIGS. 3A and 3B show a block diagram of a specific design of atransmitter unit 110 a at a base station, which utilizes MIMO and OFDMand is capable of processing data in accordance with an embodiment ofthe invention. Transmitter unit 110 a includes a TX data processor 114 athat receives and processes information bits to provide modulationsymbols, a TX channel processor 120 a that preconditions the modulationsymbols, and a number of modulators 122 that process the preconditionedmodulation symbols to generate a number of modulated signals suitablefor transmission to the terminal.

In the embodiment shown in FIG. 3A, TX data processor 114 a includes anumber of data stream processors 310 a through 310 n, one data streamprocessor 310 for each data stream to be independently coded andmodulated. (One data stream may be transmitted on all transmissionchannels, on each group of transmission channels, or on eachtransmission channel.) Each data stream processor 310 includes anencoder 312, an interleaver 314, a channelizer 316, and a symbol mappingelement 318. Encoder 312 receives the information bits for a particulardata stream and encodes the received bits in accordance with aparticular encoding scheme to provide coded bits. Channel interleaver314 interleaves the coded bits based on a particular interleaving schemeto provide diversity.

Pilot data (e.g., data of a known pattern) may also be multiplexed withthe processed information bits. The pilot data may be transmitted usingany one of the pilot transmission schemes described above. The pilotreference may be used at the terminal to characterize the forward linkand to estimate the link quality, as described above.

If code division multiplexing is utilized, channelizer 316 receives andchannelizes the interleaved packet data and the pilot data on theirassigned code channels. Symbol mapping element 318 then maps thechannelized packet and pilot data into modulation symbols for theselected transmission channels.

As shown in FIG. 3A, the data encoding and interleaving by each datastream processor 310 may be achieved based on a respective codingcontrol that identifies the specific coding and interleaving schemes tobe used for the data stream. The symbol mapping by each data streamprocessor 310 may also be achieved based on a respective modulationcontrol that identifies the specific modulation scheme to be used forthe data stream. The specific coding and modulation scheme used for eachdata stream may be selected based on the forward link quality (e.g., asreported by the terminal)

In one type of coding and modulation scheme, the coding for each datastream is achieved by using a fixed base code and adjusting thepuncturing to achieve the desired code rate, as supported by the SNRs ofthe transmission channels used to transmit the data stream. The basecode may be a Turbo code, a convolutional code, a concatenated code, orsome other code. The base code may also be of a particular rate (e.g., arate 1/3 code). For this type of scheme, the puncturing may be performedafter the interleaving to achieve the desired code rate.

Symbol mapping element 318 can be designed to group sets of channelizedbits to form non-binary symbols, and to map each non-binary symbol intoa point in a signal constellation corresponding to the modulation schemeselected for use. The modulation scheme may be QPSK, M-PSK, M-QAM, orsome other scheme. Each mapped signal point corresponds to a modulationsymbol. One modulation scheme may be selected for each transmissionchannel used to transmit the data stream, or a common modulation schememay be used for all transmission channels used to transmit the datastream.

The encoding, interleaving, and symbol mapping at transmitter unit 110 acan be performed based on numerous schemes. Some coding and modulationschemes are described in the following patent applications, which areall assigned to the assignee of the present application and incorporatedherein by reference:

-   -   U.S. patent application Ser. No. 09/532,492, entitled “HIGH        EFFICIENCY, HIGH PERFORMANCE COMMUNICATIONS SYSTEM EMPLOYING        MULTI-CARRIER MODULATION,” filed Mar. 22, 2000;    -   U.S. patent application Ser. No. 09/776,075, entitled “CODING        SCHEME FOR A WIRELESS COMMUNICATION SYSTEM,” filed Feb. 1, 2001;    -   U.S. patent application Ser. No. 09/826,481, “METHOD AND        APPARATUS FOR UTILIZING CHANNEL STATE INFORMATION IN A WIRELESS        COMMUNICATION SYSTEM,” filed Mar. 23, 2001; and    -   U.S. patent application Ser. No. 09/854,235, entitled “METHOD        AND APPARATUS FOR PROCESSING DATA IN A MULTIPLE-INPUT        MULTIPLE-OUTPUT (MIMO) COMMUNICATION SYSTEM UTILIZING CHANNEL        STATE INFORMATION,” filed May 11, 2001.

The modulation symbols from TX data processor 114 a are provided to TXchannel processor 120 a, which is one embodiment of TX channel processor120 in FIG. 1. Within TX channel processor 120 a, amultiplexer/demultiplexer 322 receives and demultiplexes the modulationsymbol from data stream processors 310 a through 310 n to the properMIMO processors 324 a through 324 l. One MIMO processor 324 is providedto perform the preconditioning for each frequency subchannel.

Each MIMO processor 324 receives a stream of modulation symbols for thespatial subchannels of a particular frequency subchannel assigned to theMIMO processor. Each MIMO processor 324 further receives an eigenvectormatrix E(k) for the frequency subchannel, which is a matrix of weightsderived from the channel response matrix H_(f)(k). Each MIMO processor324 then preconditions the modulation symbols with the eigenvectormatrix E(k), as shown in equations (5) and (6), to generate thepreconditioned modulation symbols.

The preconditioned modulation symbols from MIMO processors 324 a through324 l are provided to combiners 326 a through 326 t. One combiner 326 isprovided for each antenna at the base station. Each combiner 326receives and combines up to N_(F) preconditioned modulation symbols forN_(F) frequency subchannels for a particular antenna to form amodulation symbol vector V.

TX channel processor 120 a thus receives and processes the modulationsymbols to provide up to N_(T) modulation symbol vectors, V₁ throughV_(Nt), one modulation symbol vector for each antenna for datatransmission. Each modulation symbol vector V covers a single time slot,and each element of the modulation symbol vector V is associated with aspecific frequency subchannel having a unique subcarrier on which themodulation symbol is conveyed.

FIG. 3B is a block diagram of an embodiment of modulators 122 for OFDM.The modulation symbol vectors V₁ through V_(Nt) from TX channelprocessor 120 a are provided to modulators 122 a through 122 t,respectively. In the embodiment shown in FIG. 3B, each modulator 122includes an inverse Fast Fourier Transform (IFFT) 340, a cyclic prefixgenerator 342, and an upconverter 344.

IFFT 340 converts each received modulation symbol vector V into itstime-domain representation (which is referred to as an OFDM symbol)using IFFT. IFFT 340 can be designed to perform the IFFT on any numberof frequency subchannels (e.g., 8, 16, 32, . . . , or N_(F)). In anembodiment, for each modulation symbol vector converted to an OFDMsymbol, cyclic prefix generator 342 repeats a portion of the time-domainrepresentation of the OFDM symbol to form a “transmission symbol” for aspecific antenna. The cyclic prefix insures that the transmission symbolretains its orthogonal properties in the presence of multipath delayspread, thereby improving performance against deleterious path effects.The cyclic prefix is selected to be sufficiently long relative to theexpected amount of delay spread. The implementation of IFFT 340 andcyclic prefix generator 342 is known in the art and not described indetail herein.

The time-domain representations from each cyclic prefix generator 342(i.e., the transmission symbols for each antenna) are further processed(e.g., converted into an analog signal, modulated, amplified, andfiltered) by upconverter 344 to generate a modulated signal suitable fortransmission over the forward link. The modulated signal generated byeach modulator 122 is then transmitted from an associated antenna 124.

OFDM modulation is described in further detail in a paper entitled“Multicarrier Modulation for Data Transmission: An Idea Whose Time HasCome,” by John A. C. Bingham, IEEE Communications Magazine, May 1990,which is incorporated herein by reference.

Receiver Unit At the Terminal

FIG. 4 is a block diagram of a specific design of a receiver unit 150 aof a terminal, which is capable of receiving data in accordance with anembodiment of the invention. The transmitted signals from N_(T) antennasat the base station are received by each of N_(R) antennas 152 a through152 r at the terminal and routed to a respective demodulator 154. Eachdemodulator 154 conditions, processes, and digitizes the respectivereceived signal to provide samples, which are provided to a RX channelprocessor 156 a.

Within RX channel processor 156 a, the samples for each receive antenna152 are provided to a respective FFT processor 406, which generatestransformed representations of the received samples and provides arespective stream of modulation symbol vectors. The N_(R) streams ofmodulation symbol vectors from FFT processors 406 a through 406 r arethen provided to multiplexer/demultiplexer 408, which firstdemultiplexes the stream of modulation symbol vectors from each FFTprocessor 406 into N_(F) streams of modulation symbols, one stream foreach of the N_(F) frequency subchannels. Multiplexer/demultiplexer 408then multiplexes the N_(R) streams of modulation symbols for eachfrequency subchannel into a stream of modulation symbol vector, r, witheach vector in the stream including N_(R) modulation symbols for theN_(R) antennas. Multiplexer/demultiplexer 408 provides the N_(F) streamsof modulation symbol vectors, one stream at a time, to a spatialprocessor 410.

For each frequency subchannel, a match filter 412 within spatialprocessor 410 filters the modulation symbol vector stream r byperforming a pre-multiplication with the conjugate-transpose channelresponse matrix H_(f) ^(H), as shown above in equation (8). The channelcoefficient matrix H_(f) for the forward link may be estimated by achannel estimator 416 based on the pilot reference transmitted from thebase station. An eigenvector matrix E is then derived by a matrixprocessor 418 based on the channel coefficient matrix H_(f), as shown inequation (4). The filtered vectors from match filter 412 are furtherpre-multiplied by a multiplier 414 with the conjugate-transposeeigenvector matrix E^(H) to generate the vectors of recovered modulationsymbols y, as shown in equation (8).

A selector 420 receives the vectors of recovered modulation symbols, y,and extracts the modulation symbols corresponding to a particular datastream to be recovered. Selector 420 may then provide a number of symbolstreams, z, corresponding to a number of data streams to be recovered bythe terminal. Each symbol stream z includes recovered modulation symbolsthat correspond to, and are estimates of, the modulation symbols b afterthe symbol mapping (but prior to the preconditioning) at the basestation for a particular data stream. Each recovered symbol stream z isthen provided to a RX data processor 158 a.

Within RX data processor 158 a, the stream of recovered modulationsymbols for a particular data stream is demodulated by a demodulationelement 432 in accordance with a demodulation scheme (e.g., M-PSK,M-QAM) that is complementary to the modulation scheme used for the datastream. Demodulation element 432 further performs decovering of thedemodulated data, if the data is channelized at the base station bycovering with orthogonal codes. The demodulated data is thende-interleaved by a de-interleaver 434 in a complementary manner to thatperformed by interleaver 314, and the de-interleaved data is furtherdecoded by a decoder 436 in a complementary manner to that performed byencoder 312. For example, a Turbo decoder or a Viterbi decoder may beused for decoder 436 if Turbo or convolutional coding, respectively, isperformed at the base station. The decoded data stream from decoder 436represents an estimate of the transmitted data stream being recovered.

A CSI processor 422 receives the pilot and/or data transmission andestimates the forward link quality, as described above. For example, CSIprocessor 422 may compute a noise covariance matrix based on thereceived data and/or pilot and may further compute the SNR of atransmission channel. The SNR may be estimated similar to conventionalpilot assisted single and multi-carrier systems, as is known in the art.The frequency response for all propagation paths (i.e., H_(f)) derivedby channel estimator 416 and the forward link quality (e.g., the SNR)estimated by CSI processor 422 may comprise the CSI that is reported bythe terminal to the base station. The CSI is then provided to a TX dataprocessor 162 (see FIG. 1) for transmission back to the base station.

Referring back to FIG. 1, the CSI (e.g., H_(f) and/or SNR) determined byRX channel processor 156 is provided to TX data processor 162, whichprocesses the CSI based on a particular processing scheme. TX dataprocessor 162 further receives and process pilot data for transmissionfrom one or more antennas and on one or more frequency subchannels. Theprocessed CSI and pilot are then received and further processed by a TXchannel processor 164 and then provided to one or more modulators 154.Modulators 154 further condition and transmit the CSI and pilot on thereverse link to the base station.

At the base station, the CSI and pilot transmitted on the reverse linkare received by antennas 124, demodulated by demodulators 122, andprovided to a RX channel processor 132. RX channel processor 132processes the received pilot to derive the channel response matrix H_(r)for the reverse link. RX channel processor 132 and a RX data processor134 also process the received data in a complementary manner to thatperformed by TX channel processor 164 and TX data processor 162 torecover the

A controller 136 uses the recovered CSI to perform a number of functionsincluding (1) determining the coding and modulation scheme to be usedfor each data stream, (2) deriving the eigenvector matrix E to be usedto precondition the modulation symbols, and (3) computing thecalibration function α(ω). Controller 136 then provides the coding andmodulation control to TX data processor 114 and the preconditioningcontrol to TX channel processor 120, as shown in FIG. 3A.

In the above description, the modulation symbols are preconditioned atthe transmitter system based on the eigenvector matrix E derived fromthe channel response matrix H_(f) for the forward link. In certainembodiments or for certain transmissions in which the channel responsematrix H_(f) is not available, the modulation symbols may be transmittedwithout preconditioning. In this case, linear spatial processing may beperformed on the received symbols (for a non-dispersive MIMO channelwith flat fading) or space-time processing may be performed (for adispersive MIMO channel with frequency selective fading) at the terminalto null out the undesired signals and to maximize the received SNR ofeach of the constituent signals in the presence of noise andinterference from other signals. The ability to effectively nullundesired signals or optimize SNR depends upon the correlation in thechannel response matrix H_(f).

The spatial processing may be achieved using linear spatial processingtechniques such as a channel correlation matrix inversion (CCMI)technique, a minimum mean square error (MMSE) technique, and others. Thespace-time processing may be achieved using linear space-time processingtechniques such as a MMSE linear equalizer (MMSE-LE), a decisionfeedback equalizer (DFE), a maximum-likelihood sequence estimator(MLSE), and others. The CCMI, MMSE, MMSE-LE, and DFE techniques aredescribed in further detail in the aforementioned U.S. patentapplication Ser. Nos. 09/826,481 and 09/854,235.

For clarity, various aspects have been described for data transmissionon the forward link from the base station to the terminal. Thetechniques described herein may also be applied for data transmission onthe reverse link from the terminal to the base station.

The elements of the base station (e.g., as shown in FIGS. 1, 3A, and 3B)and the elements of the terminal (e.g., as shown in FIGS. 1 and 4) mayeach be implemented with one or more digital signal processors (DSPs),application specific integrated circuits (ASICs), processors,microprocessors, controllers, microcontrollers, field programmable gatearrays (FPGAs), programmable logic devices, other electronic units, orany combination thereof. Some of the functions and processing describedherein may also be implemented with software executed on a processor.Certain aspects of the invention may also be implemented with acombination of software and hardware. For example, the computations todetermine the channel response matrix H_(f) and the eigenvector matrix Eand the preconditioning at the base station may be performed based onprogram codes executed on a processor (e.g., controller 136 in FIG. 1).Similarly, the computations to determine the channel response matrixH_(f) and the eigenvector matrix E and the receiver processing at theterminal may be performed based on program codes executed on aprocessor.

Headings are included herein for reference and to aid in locatingcertain sections. These headings are not intended to limit the scope ofthe concepts described therein under, and these concepts may haveapplicability in other sections throughout the entire specification.

The previous description of the disclosed embodiments is provided toenable any person skilled in the art to make or use the presentinvention. Various modifications to these embodiments will be readilyapparent to those skilled in the art, and the generic principles definedherein may be applied to other embodiments without departing from thespirit or scope of the invention. Thus, the present invention is notintended to be limited to the embodiments shown herein but is to beaccorded the widest scope consistent with the principles and novelfeatures disclosed herein.

1. A method for performing data processing in a wireless communicationsystem, the method comprising: calibrating at least one communicationlink between a base station and a terminal; obtaining a channel responseestimate for the at least one communication link based on one or morepilots; and deriving at least one eigenvector from the channel responseestimate usable for spatial processing of the at least one communicationlink.
 2. The method of claim 1 wherein deriving comprises decomposing amatrix corresponding to the channel response estimate.
 3. The method ofclaim 1 further comprising performing spatial processing for datatransmissions on at least one communication link using the at least oneeigenvector.
 4. The method of claim 1 wherein performing spatialprocessing comprises transmitting a pilot on a second link, in adirection opposite the first link, using the one or more eigenvectors.5. The method of claim 1 wherein calibrating comprises calibrating basedupon a calibration function.
 6. The method of claim 5 further comprisingdetermining the calibration function based upon a forward linkcalibration function and a reverse link calibration function.
 7. Themethod of claim 1 wherein the wireless communication system implementsorthogonal frequency division modulation (OFDM), and wherein the atleast one communication link comprises a plurality of frequencysubchannels.
 8. The method of claim 1 wherein the wireless communicationsystem communication system implements multiple-input multiple-output(MIMO), and wherein the communication link comprises a plurality ofspatial subchannels.
 9. The method of claim 1 wherein the at least onecommunication link comprises a plurality of propagation paths, eachpropagation path corresponding to a path between a particular antenna atthe base station and a particular antenna at the terminal.
 10. Themethod of claim 1 wherein the channel response estimate relates tofrequency response of propagation paths used to transmit to theterminal.
 11. An apparatus for performing data processing in a wirelesscommunication system comprising: means for calibrating at least onecommunication link between a base station and a terminal; means forobtaining a channel response estimate for the at least one communicationlink based on one or more pilots; and means for decomposing the channelresponse estimate to obtain one or more eigenvectors usable for spatialprocessing of the one or more communication links.
 12. The apparatus ofclaim 11 wherein the means for calibrating comprises: means fordetermining a reverse link calibration function; and means for applyingthe forward link calibration function and a reverse link calibrationfunction.
 13. The apparatus of claim 1 wherein the means for calibratingcomprises means for calibrating based upon a calibration function. 14.The apparatus of claim 13 wherein the means for calibrating comprisesmeans for determining the calibration function based upon a forward linkcalibration function and a reverse link calibration function.
 15. Theapparatus of claim 11 wherein the wireless communication systemimplements orthogonal frequency division modulation (OFDM), and whereinthe at least one communication link comprises a plurality of frequencysubchannels.
 16. The apparatus of claim 11 wherein the wirelesscommunication system communication system implements multiple-inputmultiple-output (MIMO), and wherein the communication link comprises aplurality of spatial subchannels.
 17. The apparatus of claim 11 whereinthe channel response estimate relates to frequency response ofpropagation paths used to transmit to the terminal.
 18. A wirelesscommunication unit in a time division duplexed (TDD) communicationsystem, comprising: a processor configured to estimate characteristicsof a communication link based on a received first transmission, toderive a calibration function indicative of a difference between thefirst transmission and a second transmission, and to preconditionmodulation symbols based on weights derived at least in part from theestimated characteristics and on the calibration function; and aplurality of antennas coupled with the processor.
 19. The wirelesscommunication unit of claim 18 wherein the processor is furtherconfigured to determine the calibration function by determining adifference between a first transfer function determined based upon thefirst transmission and a second transfer function determined based uponthe second transmission.
 20. The wireless communication unit of claim 18wherein the first transmission comprise pilot symbols.
 21. The wirelesscommunication unit of claim 18 wherein the second transmission comprisesan estimate of pilot transmissions transmitted in a direction oppositeof the first transmission.
 22. The wireless communication unit of claim18 wherein the wireless communication unit implements orthogonalfrequency division modulation (OFDM), and wherein the communication linkcomprises a plurality of frequency subchannels.
 23. The wirelesscommunication unit of claim 18 wherein the communication link comprisesa plurality of spatial subchannels.
 24. The apparatus of claim 11wherein the estimated characteristics relate to frequency response ofpropagation paths used to transmit from the wireless communication unit.